The invention relates to sealing devices for rotatable shafts, where either sealed or barrier fluid is employed to generate hydrostatic and hydrodynamic forces or aerostatic and aerodynamic forces between stationary and rotary seal faces to establish separation for their non-contact operation.
Rotary fluid film face seals, also known as non-contact seals are applied to high speed and high pressure rotary shaft sealing operations, where otherwise face contact would cause excessive heat generation resulting in wear and tear of the seal faces. In a non-contact seal face operation, seal faces will separate when rotational velocity reaches lift-off speed and thus undesirable face contact is avoided.
A most successful method of generating non-contact separation between two sealing faces is by applying a shallow helical groove pattern on either one of the surfaces of the sealing faces, while the opposite sealing face remains flat and smooth. The area where the two sealing faces define a sealing clearance is labeled the sealing interface. The referred helical groove pattern applied to one of the sealing faces extends inward from the higher pressure circumference of the outer diameter to the inner end of the helical groove specified as the groove diameter.
The helical groove pattern forces fluid during shaft rotation from the higher pressure end of the sealing interface toward said groove diameter and thus drives the sealed fluid into remaining non-grooved portion of the sealing interface, thus keeping the sealing faces separated. While a certain amount of fluid will pass through the sealing interface from the side of higher pressure to the side of lower pressure, such fluid amount is considered the seal leakage, an undesirable result of the need to maintain seal face separation. The cooperation between the helical grooved area and the non-grooved area on one of the sealing faces is a most effective approach to maintain a stable gap designated the sealing clearance.
The helical groove pumping action is an effective mechanism to move fluids in between the sealing interface, regardless of whether there are pressure differences or even against pressure differentials. Moreover, even in reversed pressure differential situations, the helical groove seal still operates with adequate separation between the sealing faces, but invariably accompanied by a certain amount of leakage. Such seals are frequently used to divide two different fluids near atmospheric pressure from each other or in contingencies where intermixing of fluids must be prevented if one of them is flammable and the other one is air.
With the presence of elevated rotational velocities and pressures it becomes increasingly difficult to establish a true barrier to prevent intermixing of fluids in non-contact operation. Prior art solutions include the introduction of a third, less chemically active fluid defined as an inert fluid using Nitrogen, Carbon Dioxide or Helium to establish a barrier in a process called buffering. Said buffering can take two forms, either outside or within the sealing interface. Buffering outside the sealing interface requires incomparably larger amounts of costly inert gas due to large radial clearances requiring high flow rats of fresh, uncontaminated buffer fluid, whereas buffering inside the sealing interface, where both sealing clearances and fluid volume subjected to intermixing require much smaller amounts of buffer fluid. U.S. Pat. No. 4,523,764 provides for such purpose a buffer flow inlet as well as buffer flow outlet towards and away from the sealing interface, which as opposed to the present invention requires at least two fluid flow connections to the sealing face to establish a sealing clearance, then to recover part of the buffer fluid and more to provide for a true barrier function.
U.S. Pat. Nos. 4,212,475, 3,704,019 and 3,499,653 on the other hand, employ spiral grooves to establish a stable sealing clearance, but does not provide a solution to sealing applications, where true fluid separation or barrier is mandated.
According to the invention, buffer fluid is injected directly into and adjacent the upstream end of the sealing interface, with buffer fluid pressure slightly above that coming from the process end of the barrier unit, whereby some amount of buffer fluid is leaking towards the direction of the process, such being diametrical to that of normal interface flow and therefore terminating process fluid flow towards the sealing interface. Said amount of leakage is notably modest since it occurs through an extremely small sealing clearance of less than about 35 microns, preferably less than about 12 microns as compared to 120 microns, when buffering takes place outside the sealing interface. Resulting buffer fluid intermixing, consumption and cost being orders of magnitude smaller, when buffered inside the sealing interface, where above extremely small sealing clearances are a true result of optimum utilization of partial helical groove pattern.
Said minimal buffer fluid consumption makes it possible to minimize flow passages, which in turn facilitates the provision of more interface area for partial helical grooves, thus enhances a narrower and more stable clearance. Minimal buffer fluid consumption also makes it possible to avoid having to recover buffer fluid and having to provide flow passages for it which would once further reduce the sealing interface area needed for the advantageous benefits of the partial helical grooves.
These and many other features and attendant advantages of the invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.